1. Field of the Invention
The present invention relates to a thermal transfer ink sheet to be used for the printing of an image on printing paper by means of a thermal head, and more particularly, to a thermal transfer ink sheet provided with a back coat layer having a kinetic friction coefficient within a certain range for the elimination of printing pitch fluctuation.
2. Description of the Prior Art
There are two kinds of thermal transfer ink sheets--that of hot-melt transfer type and that of thermal dye (sublimation) transfer type. Both of them are constructed such that the substrate sheet has an ink layer (of either sublimation type or thermal transfer type) on one side thereof and a back coat layer on the other side thereof. The back coat layer prevents the thermal transfer ink sheet from sticking to the thermal head, thereby ensuring its smooth run. It is usually made of a heat-resistant resin such as silicone resin, fluorocarbon resin, acrylsilicone resin, and nitrocellulose resin, containing or not containing a slip agent such as silicone oil, fluorocarbon powder, and high-molecular weight slip polymer.
The conventional thermal transfer ink sheet has a disadvantage that it causes the printing pitch fluctuation because the back coat layer greatly varies in the kinetic friction coefficient depending on whether printing is going on or not. This holds true particularly of that of sublimation type which needs higher printing energy than that of hot-melt type and hence has a back coat layer which is formed so as to exhibit the maximum slip properties and heat resistance at the time of printing.
The printing pitch fluctuation is salient in the printing of digital signals, in which case the printing position is greatly dislocated, and in the printing of gradated images, in which case the image density greatly varies from one point to another.
FIG. 1 illustrates how the printing pitch fluctuation occurs in the printing by a line printer. FIGS. 2 and 3 show the examples of printing pitch fluctuation.
In FIG. 1, the thermal head of a line printer is schematically shown together with its nearby components. The thermal head 1 is opposite to the platen 2, with the thermal transfer ink sheet 3 interposed between them such that the thermal head 1 heats the thermal transfer ink sheet 3 through its back coat layer, thereby transferring an image onto the printing paper 4. The platen 2 turns to run the thermal ink sheet 3 and printing paper 4 in their respective directions indicated by arrows.
In the case of the printer constructed as mentioned above, the thermal head 1 have resistors whose center is slightly offset from the center of the platen 2 in anticipation of the thermal head 1 deforming during printing, (as indicated by a). During printing, the thermal head 1 receives a load in the rightward direction due to friction between the thermal head 1 and the thermal ink sheet 3. This load deforms the thermal head 1 rightward, causing the center of the resistor to coincide with the center of the platen 2, (as indicated by b). Ink transfer takes place when the resistor is at this position. However, smooth ink transfer will not take place if the back coat layer 3 of the thermal transfer ink sheet 3 varies in the kinetic friction coefficient depending on whether printing is going on or not. Such variation causes the thermal head 1 to deform in different amounts, and hence to cause the center position of the resistor to vary, depending on whether printing is going or not. This is the reason for the image pitch fluctuation.
If the kinetic friction coefficient is small when printing is going on and large when printing is not going on, the center of the resistor 1 will be at position c and position d, respectively, as shown in FIG. 1. This situation poses a problem in the case where a long line and a short line are printed alternately and repeatedly, with a blank line interposed between them, as shown in FIG. 2. When a long line is being printed, the center of the resistor is at position c in FIG. 1; however, when a short line is being printed or no printing is being performed, the center of the resistor is at position d in FIG. 1. The result is that the long lines and short lines to be printed at the same intervals (i=ii) are printed at different intervals (i&lt;ii), as shown in FIG. 2. The foregoing also applies to the printing of halftone with blanks. In this case, thick lines (iii) and thin lines (iv) appear on the printing paper, as shown in FIG. 3.
What is shown in FIGS. 2 and 3 is true of the case where the kinetic friction coefficient is large when printing is going on and small when printing is not going on. However, the situation may be reversed, in which case the printing of alternating long and short lines, with a blank line interposed between them, has uneven intervals (i&gt;ii) or the printing of halftone with blanks has thin lines (iii) and thick lines (iv).
The present invention was completed to solve the problem associated with the printing pitch fluctuation which occurs when the conventional thermal transfer ink sheet is employed. It is an object of the present invention to provide a thermal transfer ink sheet which runs smoothly without imposing unnecessary loads to the thermal head, thereby producing a high-quality transferred image free from printing pitch fluctuation even in the case of printing by digital signals.